Chapter 8: Arrays

Starting Out with C++ Early Objects Seventh Edition

by Tony Gaddis, Judy Walters, and Godfrey Muganda

Topics

8.1 Arrays Hold Multiple Values8.2 Accessing Array Elements8.3 Inputting and Displaying Array

Contents8.4 Array Initialization8.5 Processing Array Contents8.6 Using Parallel Arrays

8-2

Topics (continued)

8.7 The typedef Statement8.8 Arrays as Function Arguments8.9 Two-Dimensional Arrays8.10 Arrays with Three or More

Dimensions8.11 Vectors8.12 Arrays of Class Objects

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8.1 Arrays Hold Multiple Values

• Array: variable that can store multiple values of the same type

• Values are stored in adjacent memory locations

• Declared using [] operatorconst int ISIZE = 5;

int tests[ISIZE];

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Array Storage in Memory

The definition int tests[ISIZE]; // ISIZE = 5

allocates the following memory

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Element 0 Element 1 Element 2 Element 3 Element 4

Array Terminology

In the definition int tests[ISIZE];

– int is the data type of the array elements– tests is the name of the array– ISIZE, in [ISIZE], is the size declarator. It

shows the number of elements in the array.– The size of an array is the number of bytes

allocated for it (number of elements) * (bytes needed for each

element)

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Array Terminology Examples

Examples:Assumes int uses 4 bytes and double uses 8

bytes

const int ISIZE = 5, DSIZE = 10;

int tests[ISIZE]; // holds 5 ints, array // occupies 20 bytes

double volumes[DSIZE];// holds 10 doubles // array is 80 bytes

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8.2 Accessing Array Elements

• Each array element has a subscript, used to access the element.

• Subscripts start at 0

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subscripts 0 1 2 3 4

Accessing Array Elements

Array elements (accessed by array name and subscript) can be used as regular variables

tests[0] = 79;cout << tests[0];cin >> tests[1];tests[4] = tests[0] + tests[1];cout << tests; // illegal due to // missing subscript

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0 1 2 3 4 tests

8.3 Inputting and Displaying Array Contents

cout and cin can be used to display values from and store values into an array

const int ISIZE = 5;

int tests[ISIZE]; // Define 5-elt. array

cout << "Enter first test score ";

cin >> tests[0];

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Array Subscripts

• Array subscript can be an integer constant, integer variable, or integer expression

• Examples: Subscript is

cin >> tests[3]; int constant

cout << tests[i]; int variable

cout << tests[i+j]; int expression

What happens if subscript is not an integer???

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Inputting and Displaying All Array Elements

To access each element of an array– Use a loop (usually a for loop)– Let loop control variable be array

subscript– A different array element will be

referenced each time through loop

for (i = 0; i < 5; i++) cout << tests[i] << endl;

Note difference between array location and contents!!!

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Getting Array Data from a File

const int ISIZE = 5, sales[ISIZE]; ifstream dataFile; datafile.open("sales.dat"); if (!dataFile) cout << "Error opening data file\n"; else { // Input daily sales for (int day = 0; day < ISIZE;day++) dataFile >> sales[day]; dataFile.close(); }

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No Bounds Checking

• There are no checks in C++ that an array subscript is in range – so no messages

• An invalid array subscript can cause program to overwrite other memory

• Example:const int ISIZE = 3;int i = 4;int num[ISIZE];num[i] = 25;

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num

[0] [1] [2]25

Off-By-One Errors• Most often occur when a program

accesses data one position beyond the end of an array, or misses the first or last element of an array.

• Don’t confuse the ordinal number of an array element (first, second, third) with its subscript (0, 1, 2)

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8.4 Array Initialization• Can be initialized during program

execution with assignment statements tests[0] = 79; tests[1] = 82; // etc.

• Can be initialized at array definition with an initialization list const int ISIZE = 5;int tests[ISIZE] = {79,82,91,77,84};

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Partial Array Initialization

• If array initialized at definition with fewer values than size of array, remaining elements set to 0 or NULL

int tests[ISIZE] = {79, 82};

• Initial values used in order; cannot skip over elements to initialize noncontiguous range

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79 82 0 0 0

Start at element 0 or 1?• May choose to declare arrays to be

one larger than needed. Allows use of element with subscript 1 as ‘first’ element, etc. & may minimize off-by-one errors.

• Element with subscript 0 is not used.• Most often done when working with

ordered data, e.g., months of the year or days of the week

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Implicit Array Sizing• Can set array size by size of the

initialization list short quizzes[]={12,17,15,11};

• Must use either array size declarator or initialization list when array is defined

short quizzes[4]={12,17,15,11};

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12 17 15 11

8.5 Processing Array Contents

• Array elements can be – treated as ordinary variables of same

type as array– used in arithmetic operations, in

relational expressions, etc.

• Example: if (principalAmt[3] >= 10000) interest = principalAmt[3] * intRate1; else interest = principalAmt[3] * intRate2;

8-20

Using Increment & Decrement Operators with Array Elements

When using ++ and -- operators, don’t confuse the element with the

subscript

tests[i]++; // adds 1 to tests[i]

tests[i++]; // increments i, but has // no effect on contents

// of tests

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Copying One Array to Another

• Cannot copy with an assignment statement:

tests2 = tests; //won’t work

• Must instead use a loop to copy element-by-element:

for (int indx=0; indx < ISIZE; indx++)

tests2[indx] = tests[indx];

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Are Two Arrays Equal?• Like copying, cannot compare in a single

expression:if (tests2 == tests)

• Use a while loop with a boolean variable:bool areEqual=true;

int indx=0;

while (areEqual && indx < ISIZE)

{ if(tests[indx] != tests2[indx])

areEqual = false;

index ++;

}

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Sum, Average of Array Elements• Use a simple loop to add together

array elementsfloat average, sum = 0;

for (int tnum=0; tnum< ISIZE; tnum++)

sum += tests[tnum];

• Once summed, average can be computedaverage = sum/ISIZE;

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Largest Array Element

• Use a loop to examine each element & find largest element (i.e., one with largest value)

int largest = tests[0]; for (int tnum = 1; tnum < ISIZE; tnum++) { if (tests[tnum] > largest) largest = tests[tnum]; } cout << "Highest score is " << largest;

• A similar algorithm exists to find the smallest element

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Partially-Filled Arrays

• The exact amount of data (and, therefore, array size) may not be known when a program is written.

• Programmer makes best estimate for maximum amount of data, sizes arrays accordingly. A sentinel value can be used to indicate end-of-data.

• Programmer must also keep track of how many array elements are actually used

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C-Strings and string Objects

Can be processed using array name – Entire string at once, or – One element at a time by using a

subscript

string city;cout << "Enter city name: ";cin >> city;

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'S' 'a' 'l' 'e' 'm'

city[0] city[1] city[2] city[3] city[4]

8.6 Using Parallel Arrays

• Parallel arrays: two or more arrays that contain related data

• Subscript is used to relate arrays

– elements at same subscript are related

• The arrays do not have to hold data of the same type

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Parallel Array Example

const int ISIZE = 5; string name[ISIZE]; // student namefloat average[ISIZE]; // course averagechar grade[ISIZE]; // course grade

8-29

01234

01234

01234

name average grade

Parallel Array Processingconst int ISIZE = 5;

string name[ISIZE]; // student name

float average[ISIZE]; // course average

char grade[ISIZE]; // course grade

...

for (int i = 0; i < ISIZE; i++)

cout << " Student: " << name[i]

<< " Average: " << average[i]

<< " Grade: " << grade[i]

<< endl;

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8.7 The typedef Statement

• Creates an alias for a simple or structured data type

• Format:typedef existingType newName;

• Example:typedef unsigned int Uint;Uint tests[ISIZE]; // array of

// unsigned ints

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Uses of typedef

• Used to make code more readable• Can be used to create alias for array

of a particular type

// Define yearArray as a data type// that is an array of 12 intstypedef int yearArray[MONTHS];

// Create two of these arraysyearArray highTemps, lowTemps;

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8.8 Arrays as Function Arguments

• Referred to as Pass by Pointer– Equivalent to Pass by Reference (Address)

• To define a function with an array parameter, use empty [] to indicate array argument

• To pass array to a function, just use array name

// Function prototype void showScores(int []);

// Function header void showScores(int tests[])

// Function call showScores(tests);

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Passing an Array Element• Passing a single array element to a

function is no different than passing a regular variable of that data type

• Function does not need to know that the value it receives is coming from an array

displayValue(score[i]); // call

void displayValue(int item) // header { cout << item << endl; }

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Passing an Entire Array

• Use array name, without any brackets, as the argument

• Can also pass the array size so the function knows how many elements to process

showScores(tests, 5); // call void showScores(int[], int); // prototype void showScores(int A[], int size)

// header8-35

Using typedef with a Passed Array

Can use typedef to simplify function prototype and heading// Make intArray an integer array// of unspecified sizetypedef int intArray[];

// Function prototypevoid showScores(intArray, int);

// Function headervoid showScores(intArray tests,int size)

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Modifying Arrays in Functions

• Array parameters in functions are similar to reference variables– No copy, share array memory location

• Changes made to array in a function are made to the actual array in calling function

• Must be careful that an array is not inadvertently changed by a function

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8.9 Two-Dimensional Arrays

• Can define one array for multiple sets of data

• Like a table in a spreadsheet

• Use two size declarators in definition

int exams[4][3];

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Number of rows

Number of cols

Two-Dimensional Array Representation

int exams[4][3];

Use two subscripts to access element exams[2][2] = 86;

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exams[0][0] exams[0][1] exams[0][2]

exams[1][0] exams[1][1] exams[1][2]

exams[2][0] exams[2][1] exams[2][2]

exams[3][0] exams[3][1] exams[3][2]

columns

rows

Initialization at Definition

• Two-dimensional arrays are initialized row-by-rowint exams[2][2] = { {84, 78},

{92, 97} };

• Can omit inner { }

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84 78

92 97

Passing a Two-Dimensional Array to a Function

• Use array name as argument in function callgetExams(exams, 2);

• Use empty [] for row and size declarator for col in the prototype and header

// Prototype, where NUM_COLS is 2 void getExams(int[][NUM_COLS], int); // Headervoid getExams (int exams[][NUM_COLS], int rows)

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Using typedef with aTwo-Dimensional Array

Can use typedef for simpler notation

typedef int intExams[][2];...

// Function prototypevoid getExams(intExams, int);

// Function header void getExams(intExams exams, int rows)

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2D Array Traversal

• Traversal: to visit every element in a data structure

• Use nested loops, one for row and one for column, to visit each array element.

• Accumulators can be used to sum (process) elements row-by-row, column-by-column, or over the entire array.

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Example: 2D array

// Fill array by rows

int A[3][5],r,c; //3 rows,5 columns

for (r=0;r<3;++r)

for (c=0;c<5;c++)

{ cin >> A[r][c];}

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Example: 2D array

// Print array by rows

int A[3][5],r,c;//3 rows,5 columns

for (r=0;r<3;++r)

{for (c=0;c<5;c++) { cout << A[r][c];}

cout << ‘\n’;

}

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Example: 2D array

// Sum & Avg values in 3x5 array

int A[3][5],r,c,sum = 0;

float avg;

for (r=0;r<3;++r)

{for (c=0;c<5;c++) {sum += A[r][c]}

}

avg = sum/15.0;

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Processing by Columns

// Fill array by columns

int A[3][5],r,c; //3 rows

//5 columns

for (c=0;r<5;++c)

for (r=0;c<3;r++)

{ cin >> A[r][c];}

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CMPS 1043

•Stop Here in Chapter 8

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8.10 Arrays with Three or More Dimensions

• Can define arrays with any number of dimensionsshort rectSolid(2,3,5);

double timeGrid(3,4,3,4);

• When used as parameter, specify size of all but 1st dimensionvoid getRectSolid(short [][3][5]);

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8.11 Vectors

• Holds a set of elements, like an array

• Flexible number of elements - can grow and shrink– No need to specify size when defined – Automatically adds more space as needed

• Defined in the Standard Template Library (STL)– Covered in a later chapter

• Must include vector header file to use vectors

#include <vector>

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Vectors

• Can hold values of any type– Type is specified when a vector is defined vector<int> scores;

vector<double> volumes;

• Can use [] to access elements

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Defining Vectors

• Define a vector of integers (starts with 0 elements)vector<int> scores;

• Define int vector with initial size 30 elementsvector<int> scores(30);

• Define 20-element int vector and initialize all elements to 0 vector<int> scores(20, 0);

• Define int vector initialized to size and contents of vector finalsvector<int> scores(finals);

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Growing a Vector’s Size

• Use push_back member function to add an element to a full array or to an array that had no defined size // Add a new element holding a 75scores.push_back(75);

• Use size member function to determine number of elements currently in a vector howbig = scores.size();

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Removing Vector Elements

• Use pop_back member function to remove last element from vector scores.pop_back();

• To remove all contents of vector, use clear member function scores.clear();

• To determine if vector is empty, use empty member functionwhile (!scores.empty()) ...

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8.14 Arrays of Class Objects

• Class objects can also be used as array elements class Square{ private: int side; public: Square(int s = 1) { side = s; } int getSide() { return side; }};Square shapes[10]; // Create array of 10 // Square objects

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Arrays of Class Objects

• Like an array of structures, use an array subscript to access a specific object in the array

• Then use dot operator to access member methods of that object

for (i = 0; i < 10; i++) cout << shapes[i].getSide() << endl;

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Initializing Arrays of Objects

• Can use default constructor to perform same initialization for all objects

• Can use initialization list to supply specific initial values for each object

Square shapes[5] = {1,2,3,4,5};

• Default constructor is used for the remaining objects if initialization list is too short

Square boxes[5] = {1,2,3};

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Initializing Arrays of Objects

If an object is initialized with a constructor that takes > 1 argument, the initialization list must include a call to the constructor for that object Rectangle spaces[3] =

{ Rectangle(2,5), Rectangle(1,3), Rectangle(7,7) };

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8.13 Arrays of Structures

• Structures can be used as array elements struct Student{

int studentID;string name;

short year;double gpa;

};const int CSIZE = 30;Student class[CSIZE]; // Holds 30 // Student structures

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Arrays of Structures

• Use array subscript to access a specific structure in the array

• Then use dot operator to access members of that structure cin >> class[25].studentID;

cout << class[i].name << " has GPA " << class[i].gpa << endl;

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Chapter 8: Arrays

Starting Out with C++ Early Objects Seventh Edition

by Tony Gaddis, Judy Walters, and Godfrey Muganda